1. Field of the invention
The present invention relates to a nonlinear optical material and a nonlinear optical device and a directional coupling type optical switch using the same.
2. Description of the Related Art
In optical data processing equipment for optical exchanges, optical computers, optical interconnections, and the like, optical switches which exchange the light among waveguides by electrical signals are indispensable. As the basic form of an optical switch, the directional coupler shown in FIG. 1 for example is known. When two waveguides 1 and 1' are made to approach each other to about a wavelength of light 2, transfer of optical power occurs between the waveguides at a certain coupling length. A directional coupling type optical switch is one which controls the transfer of the optical power by causing the index of refraction of the coupling portion to change by, for example, the electro-optic effect.
The conventional directional coupler had been comprised between two waveguides on a plane, but we newly proposed a multilayer type directional coupler obtained by superposing layers of waveguides and constituting directional couplers between those layers (see Japanese Unexamined Patent Publication No. 6-82642 (Application No. 4-48961 and 5-38869)). Using this multilayer directional coupler, it becomes possible to achieve a high degree of integration of the optical circuits.
The distance over which the transfer of light occurs (i.e., coupling length) is determined by the thickness and index of refraction of the core and cladding layers of the waveguides. When fabricating a directional coupler, it is necessary to bring the waveguides close to each other by exactly the distance suitable for the coupling length. In a conventional planar type directional coupler, it had been easy to bring the waveguides into close proximity by exactly the necessary distance, but a suitable technique for a multilayer type directional coupler as we had newly proposed had not been conventionally known.
At the time of fabricating the directional coupler, it is necessary to bring the two waveguides 1 and 1' close together as shown in, for example, FIG. 1, by exactly the interval suitable for the coupling length. In the conventional planar type directional coupler, it had been easy to bring the waveguides close to each other by exactly the necessary interval, but in the multilayer type directional coupler we newly proposed, there had been no suitable technique for this.
In the case of constituting a directional coupler, the coupling constant (.kappa.) between waveguides and the difference (.beta.) between the propagation constants (.delta.) of the two waveguides become important (for example, Yariv. Introduction to Optical Electronics, 3rd edition (Holt, Rinehart and Winston, Inc.), published in Japan as Hikari Electronics no Kiso, 3rd edition (Maruzen), Chapter 13). The length (i.e., coupling length) of a directional coupler is expressed as .pi./2.kappa., while the maximum value of the transfer of optical power at the time of insertion of light into a waveguide is expressed by .kappa..sup.2 /(.kappa..sup.2 +.delta..sup.2). That is, for use as a directional coupler, it is necessary that coupling constant .kappa. be sufficiently larger than propagation constant .delta..
The conventional optical coupler had been fabricated by formation of waveguides on a substrate of an electro-optic material such as LiNbO.sub.3 by, for example, diffusion of Ti (for example, see Nishihara, Haruna, and Suhara, Optical Integrated Circuits (Ohm Co.), Chapter 10). At this time, while the electrode structures differ depending on the optical axes of the crystal, basically in the state with no electric field applied, the propagation constants .delta. are equal (.beta.=0). In two waveguides between which transfer of light occurs, by applying a voltage (i.e., giving a difference in index of refraction to the two waveguides) so as to give a difference to the propagation constants .delta., the transfer or leakage of light is suppressed and switching is performed.
When it was attempted to apply this method to a multilayer type directional coupler using a conventional polymer electro-optic material, however, difficulties occurred. In a polymer electro-optic material for poling, for example, an electric field orientation treatment is performed to impart nonlinear optical characteristics, but the electrodes used for the electric field orientation treatment are used as they are as the electrodes for bringing out the electro-optic effect. In this case, the change in the index of refraction due to the electro-optic effect becomes substantially equal for all layers and it is not possible to give a difference in propagation constants .delta. to two waveguides by applying a voltage. Further, even in the case of using the third-order nonlinear optical effect or the thermo-optic effect, in a multilayer type directional coupler, it is difficult to change the index of refraction for just one waveguide.
A second-order nonlinear optical material, however, is only realized by a substance which does not have inverted symmetry, and therefore, the polymer has to be subjected to a poling treatment to orient the molecules in one direction. Such molecularly oriented polymers, however, have the defects of a gradual weakening of the orientation and a smaller nonlinear effect due to the heat motion of the molecular chains. To prevent this relaxation of orientation, it is effective to use a polymer with a high glass transition temperature. Use of a polyimide, which is a polymer with a high heat resistance, is being looked at. From this viewpoint, Wu et al. of Lockheed have obtained a diffusion type nonlinear optical polymer using a polyimide as a host by mixing molecules with a large nonlinear optical effect into polyamic acid and performing polyimidization while performing a poling treatment (J. W. Wu et al., Appl. Phys. Lett. 58. 225 (1991)). This material does not exhibit attenuation of the nonlinear optical response (i.e., electro-optic effect: EO effect) due to the relaxation of orientation even in the face of heat treatment of 150.degree. C. for 10 hours or more, but the EO coefficient is a few pm/V (EO coefficient of LiNbO.sub.3 is 30 pm/V), and therefore, the material is not practical. The EO coefficient is small in this material probably because the concentration of the guest nonlinear optical molecules in the diffusion type material cannot be made that large.
The present inventors previously proposed the use of an azobenzene dye in the nonlinear optical material (see Japanese Unexamined Patent Publication No. 5-196214 (Application No. 4-7607)). However, when the azobenzene dye is used as a waveguide type electrooptical device (e.g., optical switch, light modulator), the wavelength (.lambda.) of light usable is limited to a long wavelength of 1 .mu.m or more due to the high absorption loss because the azobenzene dye has a wide shape of the light absorption spectrum.